Sixième partie. — Etude par spectrométrie Mössbauer des alliages métalliques amorphes et des semi-conducteurs amorphes

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Enthalpies of formation of L12 intermetallics derived from heats of reordering

A new method is proposed for estimating the enthalpies of formation of L12 (fcc-ordered) intermetallics from the heat release measured during ordering of their disordered polymorphs. The method is applied to Cu3Au, Ni3Al, and Ni3Si. The resulting estimates of enthalpies of formation are close to values obtained by high temperature dissolution calorimetry. They also appear to be more precise than estimates based on Miedema's correlations provided that care is taken to account properly for the magnetic and lattice stability contributions to the formation enthalpies in the ordered and disordered states

Etude par spectrométrie Mössbauer d'alliages Fe-Si-B amorphes

Amorphous Fe100-x-ySi yBx alloys have been studied by Môssbauer Spectroscopy. The fitting methods and some possible origins of the asymmetric broadenings of the peaks are discussed. A comparison of the hyperfine field distributions in alloys with and without silicon suggests that iron and silicon are located in similar sites (for the studied y values, y ≤ 9) while boron occupies interstitial sites with ≃ 9 iron atoms nearest neighbours. Crystallisation steps and mechanisms have been studied in Fe82Si3B15 and Fe76Si9B15 alloys. They differ from those found in Fe-B alloys because metastable borides are not observed.Des alliages amorphes Fe100-x- ySiyBx ont été étudiés par spectrométrie Môssbauer. Les méthodes de calcul et les origines possibles des dissymétries des spectres sont discutées. La comparaison des distributions de champ hyperfin des alliages avec et sans silicium suggère que le silicium et le fer occupent des sites semblables (pour les valeurs de y étudiées, y ≤ 9) alors que le bore occupe une position interstitielle avec ≃ 9 atomes de fer proches voisins. Les alliages Fe-B sont fortement stabilisés par le silicium. Les étapes et les mécanismes de cristallisation d'alliages Fe82Si3B 15 et Fe76Si9B15 ont été précisés. Ils diffèrent de ceux observés dans les alliages Fe-B car il ne se forme pas de borures métastables

Universal Cubic Eigenvalue Repulsion for Random Normal Matrices

Random matrix models consisting of normal matrices, defined by the sole constraint $[N^{\dag},N]=0$, will be explored. It is shown that cubic eigenvalue repulsion in the complex plane is universal with respect to the probability distribution of matrices. The density of eigenvalues, all correlation functions, and level spacing statistics are calculated. Normal matrix models offer more probability distributions amenable to analytical analysis than complex matrix models where only a model wth a Gaussian distribution are solvable. The statistics of numerically generated eigenvalues from gaussian distributed normal matrices are compared to the analytical results obtained and agreement is seen.Comment: 15 pages, 2 eps figures. to appar in Physical Review

Debye temperature of disordered bcc-Fe-Cr alloys

Debye temperature, TD, of Fe100-xCrx disordered alloys with 0<x<99.9 was determined from the temperature dependence of the centre shift of 57Fe Mossbauer spectra recorded in the temperature range of 80-300K. Its compositional dependence shows an interesting non-monotonous behaviour. For 0<x<~45 as well as for ~75<x<~95 the Debye temperature is enhanced relative to its value of a metallic iron, and at x=~3 there is a local maximum having a relative height of ~12% compared to a pure iron. For ~45~95 the Debye temperature is smaller than the one for the metallic iron, with a local minimum at x=~55 at which the relative decrease of TD amounts to ~12%. The first maximum coincides quite well with that found for the spin-waves stiffness coefficient, D0, while the pretty steep decrease observed for x>~95 which is indicative of a decoupling of the probe Fe atoms from the underlying chromium matrix is likely related to the spin-density waves which constitute the magnetic structure of chromium in that interval of composition. The harmonic force constant calculated from the Debye temperature of the least Fe-concentrated alloy (x>99.9) amounts to only 23% of the one characteristic of a pure chromium.Comment: 15 pages, 7 figures, 26 reference

Topological correlations in soap froths

Correlation in two-dimensional soap froth is analysed with an effective potential for the first time. Cells with equal number of sides repel (with linear correlation) while cells with different number of sides attract (with NON-bilinear) for nearest neighbours, which cannot be explained by the maximum entropy argument. Also, the analysis indicates that froth is correlated up to the third shell neighbours at least, contradicting the conventional ideas that froth is not strongly correlated.Comment: 10 Pages LaTeX, 6 Postscript figure

STRUCTURAL DESCRIPTION OF TRANSITION METAL-METALLOID GLASSES

On propose une description structurale des verres métal de transition-métalloïde qui s'appuie sur les opérations structurales qui permettent de générer les composés cristallins. Une démixtion à l'échelle microscopique s'introduit naturellement et ses conséquences sur la structure sont discutées. On apporte des arguments expérimentaux en faveur d'une telle description qui pourrait fournir un moyen de prévoir certaines propriétés des verres métalliques.A structural description of transition metal-metalloid glasses is proposed. It is based on the structural operations which allow generating the crystalline counterparts. A demixion at a microscopic level is naturally introduced whose consequences on the structure are discussed. Experimental arguments in favour of such a description are given which should provide a basis for the prevision of some properties of metallic glasses